1. Field of the Invention
The present invention relates to a recessed screw having a recess formed in the head thereof and adapted to receive a driver bit, and, more particularly, to an improved shape of the recess.
The invention further includes a driver bit for use with and engageable with the recessed screw having the improved recess.
2. Description of Prior Art
The most popular recess formed in the screw head for engagement with a driver bit is the so-called Phillips type, in which the recess is of a crossed shape. Generally, the cross-shaped recess has torque transmitting walls which are slanted inwardly towards a bottom of the recess. These slanted walls of the cross-shaped recess are likely to cause the bit to disengage from and slip out of the recess when a large torque is imparted to the bit. In addition to such a problem, a sufficiently large torque cannot be applied to the driver bit because it has only four blades or ridges.
It also has been proposed to employ recesses of shapes including hexagonal recess, in place of the cross-shaped recess, in order that a higher torque can be imparted to the modified recess than in the case of the cross-shaped recess. An example of the modified recesses is shown in FIGS. 9 and 10, in which a screw head 1 has a driver bit-engaging recess 2 composed of two holes square in transverse cross section, as indicated by the phantom lines "A" and "B". The square holes are superimposed one upon another and disposed symmetrically and with a phase shift of 45.degree. around an axis of the screw. Eight corner grooves 3 are each defined by a pair of side walls 4 and 4, which are perpendicular to each other and extend towards a bottom 5 of the recess 2 ( see for example U.S. Pat. Nos. Nos. 3,604,305 and 4,384,812).
It is however noted that the screw having the modified recess 2 as described above has also the side walls 4 which are, similarly to the cross-shaped recess, slanted inwardly towards the bottom 5. Such slanted walls will facilitate the sticking or biting of the driver bit in said recess, but will inevitably cause the so-called "come-out" problem.
It may be possible to form the side walls 4 to extend in parallel with the screw axis so as to avoid the disengagement from the bit. This hypothetical shape will be effective to transmit a higher torque giving a stronger fastening force. However, a slight clearance necessary for the recess to smoothly receive the driver bit is disadvantageous in that the bit will be held in the recess 2 in an engaged state. When the screw is self-drilling or self-tapping screw which is driven at much higher speeds by the driver bit, bit will probably slip out of the hypothetic recess. Further, the initial insertion of the driver bit into the recess will be difficult so that the threading operation is not easily performed by an automatic tool.